1. Field of the Invention
The present invention relates to a grain-oriented steel sheet having a so-called {110}&lt;001&gt; Goss texture and used as core material for transformers and other electrical machinery and apparatus. The present invention also relates to a process for producing the grain-oriented silicon steel sheet.
2. Description of the Related Art
One of the recent trends in the production of electrical machinery and apparatus has been the increased use of silicon steels having lower watt loss, which may be representated by the expression W.sub.17/50, i.e., energy lost under a magnetizing force of 17 kG at 50 Hz. In addition, endeavors have been made to improve magnetic flux density, which may be represented b B.sub.10, i.e., the magnetic flux density under the application of a magnetizing force of 1000 A/m.
To improve the watt-loss characteristic, endeavors heretofore have been made to orient the crystal grains closely to the complete Goss orientation. In addition, attention has been paid to the crystal-grain size, resistivity, and the surface coating, all of which influence the watt-loss characteristic.
Nevertheless, there are only a few methods, at present, which can provide a low watt loss of W.sub.17/50 &lt;1.0 w/kg. Further, these methods are either unsuited for industrial-scale operations or produce products unsuited for use for certain kinds of the electrical machinery and apparatus.
For example, one method proposes to increase the silicon content so as to raise the resistivity. This, however, impairs the workability. Thus, the maximum silicon content is restricted by the workability.
Other methods aim at improving the watt-loss characteristics by imparting a tensional force. Toward this end, Japanese Examined Patent Publication (Kokoku) No. 51-12451 discloses a method for improving the quality of a forsterite coating, and Japanese Examined Patent Publication No. 53-28375 discloses a method for top coating. While the tensional force can be increased with an increase in thickness of these coatings, the improvement attained by the effects of tensional force is restricted by the space factor of the transformer core as the space factor is lessened with the increase in the coating thickness. Still other methods aim at improving the watt-loss characteristics include the marking-off methods, disclosed in Japanese Examined Patent Publication No. 58-5968, and the laser-irradiation method, disclosed in Japanese Examined Patent Publication No. 58-26405. These, however, can only be applied for grain-oriented silicon steels which do not undergo stress-relief annealing, because the improvement attained by the marking-off and the like is lost by stress-relief annealing. Also well known as a method for watt-loss reduction is grain-refining. Japanese Unexamined Patent Publication (Kokai) No. 57-41326 discloses watt-loss reduction of thin sheets by grain-refining. The grain-refinement, however, generally makes a high magnetic flux density difficult, i.e., both a high magnetic flux density and low watt-loss cannot be obtained in the grain-refining method.